1. Field of the Invention
The present invention relates to an optical modulator mainly used for optical communication. In particular, the present invention relates to a semiconductor Mach-Zehnder optical modulator and a control method therefor.
2. Description of the Background Art
Recently, in order to realize a super high-speed and wideband optical communication network system, there is high expectation for an optical modulator of an external modulation method (external modulator). Particularly, in order to enable long-distance transmission of optical signals, development of a Mach-Zehnder (MZ) optical modulator using LiNbO3 (lithium niobate; LN) (MZ type LN optical modulator) and having excellent high-speed modulation properties and dispersion resistance in a wideband is under way.
Generally, a Mach-Zehnder device includes two parallel phase modulated optical waveguides, an electrode for applying a voltage to these waveguides, and two Y branches on an input side (first stage) and an output side (second stage). When light enters an input optical waveguide, the light branched in the Y branch in the first stage propagates respectively in the two parallel phase modulated optical waveguides by the same distance, is recoupled in the Y branch in the second stage, and is emitted from an output signal optical waveguide.
In a state where a voltage is not applied to the phase modulated optical waveguides, there is no change in the refractive index of the waveguides, and hence a phase difference of the light propagating in the two phase modulated optical waveguides is 0°, that is, the coupled signal has the same signal size as that of the signal before being coupled. Conversely, when a voltage is applied so that the phase difference between the two guided waves becomes 180°, the coupled signals serve to negate each other. Thus, the output signal can be used as a modulation signal in an ON or OFF state according to the intensity of light, by changing the phase difference of the light propagating in the two optical waveguides, by applying a voltage.
In the MZ type LN optical modulator, the operating point fluctuates due to temperature drift, DC drift, or the like. Therefore, to compensate for this, a bias voltage may be applied to an optical modulator optical waveguide.
In conventional semiconductor Mach-Zehnder optical modulators, there is one having a structure in which, for example, a semiconductor Mach-Zehnder device, a laser device as a light source, an optical coupling lens, an optical fiber, an isolator, a temperature controlling Peltier element, and an automatic optical output controlling monitor photodiode device (hereinafter, referred to as “monitor PD”) are hybrid-integrated. As described above, the semiconductor Mach-Zehnder device functions as a shutter, which transmits or intercepts laser beams incident on the waveguide, by using a change in the refractive index of the waveguide produced in proportion to the applied voltage. On the reception side, the intensity of light transmitted through the optical fiber line is identified, to reproduce the light in a “high” or “low” electric signal.
In the semiconductor Mach-Zehnder optical modulator, due to temperature characteristics peculiar firstly to the semiconductor Mach-Zehnder device, and also to built-in functional devices such as the laser device and the isolator, modulation outputs change considerably due to not only ambient temperature but also to generation of heat in internal devices. Consequently, it is necessary to compensate for the characteristic fluctuations due to temperature change. Therefore, temperature control has been heretofore performed by a Peltier element. That is, the temperature change in a thermistor resistance is fed-back to perform the temperature control using endothermic and exothermic heat of the Peltier element. As a result, optical propagation loss inside the semiconductor Mach-Zehnder device, and a decrease in the optical output of the laser device can be suppressed.
Japanese Unexamined Patent Publication No. 2003-233047 discloses a Mach-Zehnder type LN optical modulator, in which a bias voltage is applied to the optical waveguide in order to compensate for a change in the operating point due to temperature drift, DC drift, or the like. Specifically, feed-back control is performed for controlling the bias voltage to be applied to the optical waveguide based on the intensity of the output light of the monitoring optical waveguide. As a result, the change in the operating point voltage is compensated for.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2003-233047
Furthermore, Japanese Unexamined Patent Publication No. Hei 6-43411 discloses a technique for controlling the bias voltage to be applied to the optical waveguide to control the operating point voltage of the optical modulator, as in the invention disclosed in Japanese Unexamined Patent Publication No. 2003-233047.
[Patent Document 2] Japanese Unexamined Patent Publication No. H06-43411
The temperature control method using the Peltier element, and the automatic optical output control method for monitoring the optical output intensity at the back of the laser device are used in many optical coupling modules. However, these methods cannot compensate for an optical loss when imperfect alignment occurs in an optical coupling section between the laser device and the Mach-Zehnder device. Moreover, complete compensation is not possible with respect to variable factors such as temperature characteristics and a change with the lapse of time, peculiar to integrated functional devices such as the Mach-Zehnder device and the isolator. Accordingly, there is a problem in that a sufficient effect cannot be obtained as a means for compensating for fluctuations in the modulation output. Particularly in a hybrid module in which functions and density are both highly integrated, such as in the semiconductor Mach-Zehnder modulator, characteristic compensation with respect to complex variable factors is not easy.
Here, optical loss when imperfect alignment occurs in the optical coupling section is taken into consideration. FIG. 5 depicts a general optical coupling loss characteristic (broken line) between the laser device and the semiconductor Mach-Zehnder device in the case of using a ball lens, and an optical coupling loss characteristic (solid line) due to collimator coupling between the laser device and the semiconductor Mach-Zehnder device. As seen from FIG. 5, deterioration of the optical coupling loss (solid line) due to imperfect alignment of the semiconductor Mach-Zehnder device is larger than that of the optical coupling loss (broken line) due to the imperfect alignment of a single mode fiber. It is considered that this results from coupling of semiconductor devices having a small optical mode size. Thus, in the case of an optical coupling module (the semiconductor Mach-Zehnder modulator) which is extremely sensitive to imperfect alignment, a difference in the thermal expansion coefficient of the components, or slight imperfect alignment due to stress distortion, due to the ambient temperature of the modulator or an external force, affects the semiconductor Mach-Zehnder modulator. As a result, the loss increases abruptly, and the modulation output is likely to change.
On the other hand, for the temperature characteristic change of the functional devices such as the semiconductor Mach-Zehnder device, the quality of the suppression effect is determined based on how accurately the temperature change in the vicinity of the devices can be feed-back controlled. Therefore, it is essential to optimize the heat transfer characteristics and heat radiation design, and to perform complicated heat design and analysis. However, these methods are not to directly monitor the characteristic change of the devices, but to control the characteristic change of the devices indirectly by feed-back controlling the ambient temperature change. Therefore, a sufficient compensation effect cannot be obtained, and instability occurs in the modulation output.